SAT-392 The Role of β-arrestin2 in Bone Catabolic Response to Hyperparathyroidism In Vivo

Primary hyperparathyroidism (PHPT) is an endocrine disorder characterized by elevated parathyroid hormone (PTH) levels and hypercalcemia caused by the overactive parathyroid glands, resulting in negative impacts on the skeleton including bone loss and increased bone fragility(1). PTH binds and activates parathyroid hormone type 1 receptor (PTH1R) which primary couples to Gαs, stimulating the downstream effectors that mediate bone remodeling processes(2). PTH1R activity is regulated by arrestins, specially β-arrestin2 (β-arr2), through signal termination and receptor internalization(2). Previously, we have seen anabolic effects of hyperparathyroidism (cPTH) on trabecular bone in mice overexpressing Gαs(3). We hypothesized that increased Gαs protein levels in osteoblasts outcompete β-arr binding to PTH1R, leading to reduced signal termination and increased bone formation. To test this hypothesis, we are testing if the deletion of β-arr2 will also result in an anabolic response to cPTH in this study. The response of β-arr2 knockout (KO) mice to cPTH have yet to be documented. The hypothesis of this study is that β-arr2 KO mice treated with cPTH will exhibit anabolic effects on the trabecular bone. Nine-week-old wild-type (WT) C57BL/6 and β-arr2 KO mice were treated for 14 days with either rPTH1-34 (80ng/g/day) or saline (PBS) using micro-osmotic pumps to simulate hyperparathyroidism. There are 8 groups (n=10 per group) including both sexes, 2 genotypes (WT and KO), and 2 treatment groups (PTH and PBS). Two 30 mg/kg doses of 0.6% calcein green were administered subcutaneously to mice at 7 and 2 days prior to euthanasia to label bones. Decalcified tibiae were embedded in paraffin for histological analysis. Undecalcified tibiae were embedded in plastic for dynamic histomorphometry. Micro-computed tomography (μCT) was used to access bone microarchitecture of femurs and vertebrae followed by biomechanical testing of bone strength. The μCT data of distal femurs show that cPTH treatment increased bone volume in female KO mice (6.864 ± 2.318 vs 4.690 ± 1.555 %; P= 0.0328; n=9 per group) and maintained bone in male KO mice (13.37 ± 2.860 vs 13.38 ± 3.135; P= 0.9968, n= 10) compared to control. Histological analysis show higher osteoclastic activity in both sexes and genotypes when treated with cPTH, suggesting that the anabolic response may be at the level of osteoblasts and osteocytes. These promising results support our hypothesis that arrestin-mediated PTH receptor downregulation plays an importance role in bone weakness associated with hyperparathyroidism. These studies are important for understanding the clinical phenotype of PHPT patients and suggest that inhibition of β-arr2 in PHPT could be a path for drug therapy. References: (1) Mosekilde L. Clin Endocrinol 2008;69:1-9. (2) Ferrari SL et al., J Biol Chem 1999; 274:29968–29975 (3) Zhang L. PhD thesis University of Toronto, 2018.